1. Field of the Invention
This invention relates to the formation of a layer of metal on a substrate and, in particular, to the formation of a wetting layer at high temperature that results in improved adhesion of a subsequently formed metallization layer to the wetting layer. Most particularly, the invention relates to the formation on a semiconductor substrate of a titanium (or titanium alloy) wetting layer at high temperature, and to the subsequent formation of an aluminum (or aluminun alloy) metallization layer on the titanium wetting layer.
2. Related Art
Formation of a layer of metal on a substrate (or on other material previously formed on the substrate) is a common step in the production of some devices, such as, for example, semiconductor devices. A metal layer can be formed as part of such a device to provide an electrically conductive trace, for example. Or, metal can be formed in a via or hole previously formed in the device to provide electrical contact between electrically conductive material (e.g., electrically conductive traces or other electrically conductive regions) formed adjacent to opposite ends of the via or hole. The formation of a metal layer can be subject to several problems, such as cusping and voiding, that may degrade the electrical performance of the metal layer. In particular, metal formed in vias having a high aspect ratio (i.e., ratio of the depth of the via to the width or diameter of the via) or over steps having a relatively large height has been subject to such problems.
In some methods of forming a layer of metal on a substrate, a wetting layer is first formed on the surface on which the metal layer is to be formed. A metal material (i.e., the "primary" metal with which it is desired to form the metal layer) is then formed on the wetting layer. The material with which the wetting layer is formed (which is, itself, often a metal material) attracts the primary metal material better than would the material of the surface on which the wetting layer is formed, so that the primary metal material more completely covers the surface on which it is desired to form the metal layer.
For example, in the production of semiconductor devices, aluminum is often formed (by, for example, deposition) on a semiconductor wafer. Before formation of the aluminum on a substrate surface, a wetting layer of a suitable material (such as titanium, titanium nitride, or a composition of titanium and tungsten) can be formed on that surface so that when the aluminum metallization layer is formed, the aluminum adheres more completely and uniformly to the substrate surface than would otherwise be the case. Previously, the wetting layer has been formed at relatively low process temperatures (e.g., while the process chamber and wafer are at a temperature less than about 200.degree. C. and, frequently, about 40.degree. C.) using a standard deposition process. The low temperature has been used for several reasons. First, standard heaters (such as the heater that is part of the widely used Endura sputtering system made by Applied Materials of Santa Clara, Calif.) that are used to heat the semiconductor wafer during formation of the wetting layer are not designed for extended operation at high temperatures (e.g., the recommended maximum operating temperature for the Endura heater is 450.degree. C.). Operation of a standard heater at elevated temperatures undesirably shortens the life of the heater. Second, the wetting layer is typically formed immediately after and immediately prior to other process steps that occur at a relatively low temperature (e.g., at or below about 200.degree. C.). For example, typically, the wetting layer is formed right after a preclean step or a sputter etch step, each of which are performed at a low temperature as described above. Further, the aluminum metallization layer (or at least an initial portion of the aluminum metallization layer) formed on the wetting layer is often formed at a low temperature. (This may be done, for example, because aluminum attaches to a wetting layer of titanium better when the aluminum is deposited at a relatively low temperature.) Thus, absent a reason to do otherwise, it is desirable to form the wetting layer at a temperature that is approximately the same as the temperature at which the prior and subsequent process steps will be performed (e.g., a temperature below about 200.degree. C.), so that time is not unnecessarily spent heating and cooling the wafer between process steps.
Standard deposition processes have been modified to produce other processes for depositing a wetting layer on a semiconductor wafer. Collimated deposition and ionized metal plasma (IMP) deposition are two such processes.
In collimated (or "coherent") deposition, a collimator (honeycomb) is positioned in a sputtering chamber between the sputtering target and the wafer. The collimator directs the sputtered atoms of the wetting material in a direction perpendicular to the wafer, so that the likelihood that the atoms will fall to the bottom of a deep via, for example, is increased. Generally, collimated deposition produces a higher quality wetting layer--denser and smoother--than does the standard deposition process. However, because a collimator must be positioned in the sputtering chamber, the collimated deposition system is more complex and requires more frequent preventative maintenance than a standard deposition system. Additionally, the deposition rate of a collimated deposition process is slower than that for a standard deposition process (decreasing wafer throughput). These differences make collimated deposition a more expensive process than the standard deposition described above.
In IMP deposition, radiofrequency (RF) power is used to control the directionality of the sputtered atoms of the wetting material. IMP deposition can produce a wetting layer that is even more dense and smooth than that produced by collimated deposition. However, like collimated deposition, IMP deposition is somewhat more expensive than a standard deposition process. Further, IMP deposition is still being developed; commercial IMP deposition systems are not yet available. Even when such systems are available, the high cost of replacing existing standard deposition systems with IMP deposition systems will represent a significant deterrent to the use of IMP deposition systems.
In another previous process for forming a metal layer on a semiconductor wafer, tungsten (actually, a combination of tungsten and fluorine, WF.sub.6) is formed (e.g., deposited) as the primary metal material (i.e., as the metallization layer). Tungsten is sometimes used instead of aluminum as a metallization layer on a semiconductor wafer because, for example, tungsten can more easily fill in high aspect ratio vias. In a tungsten metallization process, titanium is first formed on the surface (which is typically silicon) on which the metal layer is to be formed. The titanium does not function as a wetting layer, but is formed, instead, because it provides better contact resistance with silicon than does tungsten. However, since titanium reacts with the fluorine that is combined with the tungsten, a barrier layer (typically made of titanium nitride, though other materials could be used) must be formed over the titanium. The tungsten (WF.sub.6) is then formed over the barrier layer. In a previous tungsten metallization process, titanium nitride has been deposited as a barrier layer while the process chamber and wafer are held at a relatively elevated temperature (e.g., the wafer can be at a temperature of about 375.degree. C.). The elevated temperature has been used to reduce film stress during deposition of the titanium nitride. In such a tungsten metallization process, the titanium (which has typically been deposited using a collimated deposition process) has been deposited at approximately the same temperature as that used for deposition of the titanium nitride. This is done not for any reason associated with the titanium deposition, but, rather, occurs incidentally so that there is no need to heat the wafer between the titanium deposition and the subsequent titanium nitride deposition.
It is desirable to provide a method of forming a metal layer on a substrate that improves upon the capability of the above-described methods to produce a high quality metal layer, e.g., a metal layer having few or no voids. In particular, it is desirable to provide a method of forming a wetting layer for use in forming a metal layer that enhances the capability to produce such a high quality metal layer and that facilitates the implementation of the process steps necessary to produce a metal layer (e.g., enlarges the process window for those steps).